If you have have a sufficient power supply for heavy inductive loads then on/off heat pumps.
Once they are on keep them on and they will heat your pool effectively.
Unfortunately one of biggest problems for large on/off heat pumps is inadequate power supply.
Inverter heat pumps are here to stay and bring benefits to customers whom have limited power supplies and if all the sales blurb is to believed they will reduce your running costs.
Inverter heat pumps are the best!